slow up vs lessen

slow up and lessen both are verbs.

Word	Noun	Adjective	Verb	Adverb
slow up	No	No	Yes	No
lessen	No	No	Yes	No

As verbs, lessen is a hypernym of slow up; that is, lessen is a word with a broader meaning than slow up:

Other hypernyms of slow up include decrease, diminish, fall.

slow up (verb)	lessen (verb)
lose velocity; move more slowly	decrease in size, extent, or range
cause to proceed more slowly	wear off or die down
become slow or slower	make become smaller